String theory, (or super-string theory when coupled with Supersymmetry), is a class of models in theoretical physics which replace zero-dimensional points (particles) in four-dimensional spacetime with one-dimensional strings in an eleven-dimensional spacetime as the fundamental building block of the universe. The elusive goal is to develop a set of equations that unify all known natural forces (gravitational, electromagnetic, weak, and strong), according to some preconceived philosophical beliefs about how they ought to be unified. Leading proponents of string theory have been Institute for Advanced Study's Edward Witten, who was a history major in college and received the Fields Medal for mathematicians (not physicists) in 1990, and Brian Greene, the author of book for popular consumption called The Elegant Universe. String theory has never been successfully used to generate a model of the universe resembling our own, with previously observed particles and forces in place.
Some physicists argue that string theory (and its alter ego M-theory) is currently the most viable candidate for a unified theory of physics which describes all forces of nature, encompassing the physics of gravity as well as quantum field theory. Major research centers include, for example, MIT, with five faculty members and numerous postdocs and graduate students working in this area. There are competing string theories: one describes closed loops that can break into open loops, and the other consists of closed loops that cannot break into open loops.
Princeton University researchers made a mathematical claim that some aspects of string theory may be related to a well-respected body of physics called "gauge theory," which has been demonstrated to underlie the interactions among quarks and gluons, the vanishingly small objects that combine to form protons, neutrons and other, more exotic subatomic particles. The discovery, say the physicists, could open up a host of uses for string theory in attacking practical physics problems. But after 30 years of research, no one has found a way to apply string theory to a practical physics problem yet.
String theory originated in 1970 when particle theorists realized that the theories developed in 1968 to describe the particle spectrum also describe the quantum mechanics of oscillating strings. Supersymmetry was introduced in 1971.
Many physicists have long been critical of string theory, with some portraying it "as anything from a mathematically obtuse minefield to a quasi-religion that has precious little to do with science." By 2000, many physicists conceded that string theory was a failure, and that it had no hope of realizing any of its goals.
String theory is not even scientific, because it is not falsifiable: string theory can be utterly false and yet there is no test that would illustrate its falsity. It must be possible to define an experiment which could contradict the theory for it to qualify as science. String theory proponent Michio Kaku replies to this glaring defect by stating that string theory may be "too robust" because it is not falsifiable, an obvious non sequitur.
Some string theorists, such as Leonard Susskind, say that string theory may not be testable but may still provide insights on multiple universes. But his book defending string theory appears by its title to be a rant against Intelligent Design: "The Cosmic Landscape: String Theory and the Illusion of Intelligent Design."
Dark Energy Confounds String Theory
The discovery of data supporting an accelerating expansion of the universe, and the accompanying dark energy hypothesis, in 1998 was a particularly damaging blow to string theory. It was just the sort of thing that string theorists hoped to explain, and yet string theory cannot explain it. As of today, string theory has attracted strong interest among theorists but has not suggested any critical experiments.
Although string theory has so far failed to make viable experimental predictions, it has proved remarkably successful at predicting new theorems in mathematics. For example, from string-theoretic considerations, Candelas, de la Ossa, Green, and Parkes conjectured the correct formula for the number of degree d rational curves in a Calabi-Yau quintic. Their formula was later rigorously proved correct by Givental and Lian, Liu, and Yau, establishing that the string-theoretic prediction was accurate. More generally, string theory has predicted a deep relationship in mathematics called "mirror symmetry" which connects seemingly unrelated topics in symplectic and complex geometry. Mirror symmetry remains an active area of mathematical research, and many highly non-trivial examples of mirror symmetry have been mathematically verified.
- Smolin, Lee. The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next (2007)part 2 online
- The Elegant Universe (Nova TV Series discussing Strings)
- "String Theory, Black Holes, and the Fundamental Laws of Nature" (Harvard@Home 2007)
- "The Official String Theory Web Site" by Patricia Schwarz (PhD Physics, Cal Tech)
- interviews with leaders in the field: John Schwarz; Sir Michael Atiyah; Ed Witten; Jim Gates; Eva Silverstein; Juan Maldacena; and Brian Greene